1. Field of the Inventions
The present inventions generally relate to a valve drive system for a four-stroke engine in which rollers in rolling contact with an intake cam and an exhaust cam are incorporated in rocker arms for opening and closing an intake and an exhaust valve.
2. Description of the Related Art
The so-called single over-head cam “SOHC” type four-stroke engine in which an intake valve and an exhaust valve are opened or closed through one camshaft, typically includes an intake rocker arm for transmitting the movement of an intake cam to the intake valve and an exhaust rocker arm for transmitting the movement of an exhaust cam to the exhaust valve.
The intake and exhaust rocker arms are supported on rocker shafts for rocking movement, respectively. The rocker shafts are disposed parallel to each other and are disposed on both sides of the camshaft. The intake rocker arm extends from the intake cam, across the rocker shaft, toward the valve stem of the intake valve. Similarly, the exhaust rocker arm extends from the exhaust cam, across the rocker shaft, toward the valve stem of the exhaust valve.
In such engines, it has been known to incorporate rollers at the ends of the rockers facing the respective cam. The rollers are in rolling contact with the intake cam and the exhaust cam, which reduces the frictional resistance produced in the contact portion of the rockers with the respective cams (for example, see Japanese Patent Application No. 2001-193426).
Other systems for such four-stroke engines have included tappet-clearance adjusting shims placed between the rocker arms and the respective valve stems for reducing friction. In this type of engine, the intake and exhaust rocker arms are slidable in the axial direction of the rocker shafts, and the shims can be removed when these rocker arms are displaced to the sides of the valve stems (see, Japanese Patent Application No. Hei 11-166449).